Antenna



Nov. 16, 1954 Filed May 29, 1953 H. J. ROWLAND ETAL ANTENNA 2 Sheets-Sheet l 1 INVENTORS, 5"" HOWARD J. ROWLAND GEORGE c. TERRELL.

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A TTOR/VE X United States Patent ANTENNA Howard J. Rowland, Attleboro, and George C. Terrell,

Foxboro, Mass., assignors to the United States of America as represented by the Secretary of the Army Application May 29, 1953, Serial No. 358,294

9 Claims. (Cl. 25033.65)

Our invention relates to a radio antenna fed from a coaxial line which is capable of radiating a circularly polarized wave. The use of an antenna fed from a coaxial line is well known. For example, one well known radio object locating system employs a dipole fed from a coaxial line to illuminate a paraboloidal reflector. The dipole is offset from the axis of the reflector and revolved to produce a conical beam in a well known manner. However, this system radiates a linearly polarized wave. In many applications in communications and radio object locating systems a circularly polarized wave is more suitable.

Accordingly, it is an object of our invention to provide an antenna which will radiate a circularly polarized wave.

Another object of our invention is to provide an antenna which will radiate circularly polarized wave and which may be readily energized from a coaxial line.

Still another object of our invention is to provide an antenna in which the polarization of the wave may be readily adjusted to a linear polarization, a circular polarization or elliptical polarization.

A still further object of our invention is to provide an antenna which is adapted to be located near the axis of a reflector and revolved to produce a conical beam.

Briefly, the antenna of our invention comprises a short section of coaxial line short circuited at one end and closed by a dielectric window at the other end. The inner conductor of this line is also the outer conductor of a coaxial line connected to the source of energy. The outer, larger line is energized by means of slots in the inner conductor (which is the outer conductor of the feed line). Polarization is changed from linear to circular by means of several pairs of lumped susceptances.

For a more complete understanding of the invention reference may be made to the accompanying drawing in which Figure l is a schematic view of the antenna of our invention used to illuminate a paraboloidal reflector; Figure 2 is a view partly in elevation and partly in section showing the antenna in more detail; Figure 3 is a view in section taken along the line 33 of Figure 2; and Figure 4 is a view in section taken along the line 4-4 of Figure 2.

Figure 1 shows a portion of a section of paraboloidal reflector 10. At the vortex of the reflector is an assembly 11 including a motor and suitable rotating joints for rotating a coaxial feed line 12. Coaxial line 12 is bent at polnts 13 and 14 and supports the antenna, generally indicated by the numeral 15, at a location slightly ofiset from the axis of the reflector. The antenna 15 is supported at its other end by hearing housing 16 which is in turn supported from the rim of the reflector by means of a plurality of guy rods 17. Since the antenna is to be rotated and is offset from the axis of rotation, it is counterwelghted by means of a weight 18 secured to the antenna by a bracket 19. When the motor in assembly 11 in energized, antenna will revolve about the axis of the reflector to produce a conical beam.

Referring now to Figure 2, coaxial line 12 consists of an inner conductor 20 and an outer conductor 21. These conductors are short circuited at the end of line 12 by means of a hollow cylindrical plug 22. At a distance of the order of a quarter of a wavelength from the end of plug 22, outer conductor 21 is provided with two diametrically opposite slots 23 and 24 (also shown in Figure 3). At the same distance from the plug 22 the inner conductor 20 is connected to outer conductor 21 by means of a shorting stub 25 which is at right angles to a line connecting slots 23 and 24. This stub 25 may be secured to outer conductor 21 by any suitable means .such as a machine screw 2'6. Spaced from and surrounding outer conductor 21 is a tubular conductor 27. Conductor 27 is mounted so as to be rotatable about its axis and may be fastened in any desired angular position. Conductor 27 comprises the outer conductor of a second coaxial line of which the inner conductor is conductor This outer coaxial line is short circuited by means of a hollow cylindrical plug 28, located approxunately opposite to plug 22, at a point on the order of a quarter of the air wavelength from the center line of slots 23 and 24. Plug 28 serves also to position conductor 27 concentrically about conductor 21 while allowing conductor 27 to be rotated about its axis. Conductor 27 is provided with a pair of metallic rod-like elements 29a which protrude into the space between conductors 21 and 27. Elements 29a may be threaded into conductor 27 and constitute adjustable lumped susceptances. As best shown in Figure 4, members 29a are opposite to each other, and when adjusted for circular polarization, a line connecting them makes an angle of 45 with shorting stub 25. Another pair of lumped susceptances 29b is located at an axial distance of approximately a quarter of a guide wavelength from pair 29a and lies in the same longitudinal plane. A further pair of susceptances 29c is located approximately a quarter of a guide wavelength from the second pair 29b and is in the same longitudinal plane. At the open end of conductor 27, there is a circular metallic plate 31, which may be secured to member 27 by any suitable means. As illustrated in Figure 2 this means may comprise a nut 32 threaded on conductor 27 and a plurality of machine screws 33 threaded into nut 32. The space between outer conductor 27 and inner conductor 21 is closed by means of dielectric window 34 which may be secured by means of a nut 35 threaded onto nut 32. Window 34 aids in positioning conductor 27 concentrically about conductor 21. A gasket 37 may be located between window 34 and conductor 21 and another gasket 36 may be placed between window 34 and nut 32. These gaskets make the seal between conductors 21 and 27 air tight so that the entire antenna and feed line may be pressurized, if desired.

In operation, coaxial line 12 is energized from any suitable source such as the source 40 shown in Figure 1 so as to propagate a wave in the TEM mode. Slots 23 and 24 together with shorting stub 25 cause the outer coaxial line consisting of members 21 and 27 to be excited in the TE11 mode. The discontinuity presented by slots 23 and 24 and shorting stub 25 may be compensated for by means of a transformer 38 surrounding inner conductor 20 approximately one half wavelength from the slots. The linearly polarized TEu wave in the outer coaxial line may be thought of as consisting of two components at right angles to each other. For circular polarization, susceptances 29a, 29b and 290 are placed at an angle of 45 to stub 25 so as to retard the phase of one component while leaving the phase of the other component unchanged.

It has been found that three pairs of susceptances placed as previously described and properly adjusted are suflicient to retard one component of the wave by As a result the two components are 90 apart in space and also 90 apart in phase. A circularly polarized wave is therefore produced and radiated through window 34. \The change in impedance due to window 34 may be compensated for by means of a transformer comprising a metallic band 39 placed around conductor 21.

Figure 4 shows conductor 27 and susceptances 29a, 29b and 290 angularly positioned so as to resolve the linearly polarized wave into two components. If conductor 27 be positioned so that the susceptances 29a, 29b and 29c are parallel to shorting stub 25, the linearly polarized wave will be uniformly retarded and a linearly polarized wave will be radiated. If the angle between the susceptances and shorting stub 25 is between 0 and 45 an elliptically polarized wave will be radiated. At one of the 45 positions a right hand circularly polarized wave will be produced, while at the other 45 position, a left hand circularly polarized wave will be produced.

While a specific embodiment of the invention has been described, it is obvious that numerous modifications can be made within the scope of the invention.

What we claim as our invention is:

1. An antenna for radiating a circularly polarized wave comprising: a coaxial line including inner and outer conductors, said outer conductor being terminated at the point at which energy is to be radiated; means for exciting said line to propagate a linearly polarized Wave; and means Within said coaxial line for changing the polarization from linear to circular.

2. An antenna for radiating a circularly polarized wave comprising: a coaxial line including inner and outer conductors, said outer conductor being terminated at the point at which energy is to be radiated; means for exciting said line to propagate a linearly polarized wave; and a plurality of pairs of diametrically opposite rod-like elements, constituting lumped susceptances, adjustably secured to the inner surface of the outer conductor of said coaxial line for changing the polarization from linear to circular.

3. An antenna according to claim 2 further comprising a metallic reflector surrounding said outer conductor at its termination.

4. An antenna according to claim 3 further comprising a dielectric window for closing the space between said inner and outer conductors in the plane of the termination of said outer conductor.

5. An antenna for radiating a circularly polarized wave comprising: an open ended, hollow, conductive tube; a coaxial feed line extending into said tube on its center line and terminating at one end of said tube; a first cylindrical plug for short circuiting said tube to the outer conductor of said coaxial line at said one end of said tube; a second cylindrical plug for short circuiting the end of said coaxial line; a pair of slots in the outer conductor of said coaxial line located approximately one quarter of a wavelength from said second plug; and means on said conductive tube for changing the wave from a linear to a circular polarization, whereby a circularly polarized wave will be radiated from the other end of said tube.

6. An antenna according to claim 5 in which the means 4 for changing the polarization comprises a plurality of pairs of metallic rod-like elements, constituting lumped susceptances, adjustably secured to the inner surface of said conductive tube.

7. An antenna according to claim 6 further comprising a metallic reflector surrounding said other end of said tube.

8 Apparatus for exciting a circularly polarized wave in a coaxial line comprising a first coaxial line in which the TEM mode is propagated; a second coaxial line concentric with said first line in which the inner conductor of said second line is the outer conductor of said first line; slot means in the outer conductor of said first line for exciting a linearly polarized Wave in second line; and a plurality of lumped susceptances on the inner surface of the outer conductor of said second line for changing the polarization from linear to circular.

9. Apparatus for exciting the TEu mode in a coaxial line comprising a first coaxial line in which the TEM mode is propagated; a second coaxial line concentric with said first line in which the inner conductor of said second line is the outer conductor of said first line; a pair of diametrically opposite slots in the outer conductor of said first line; a shorting stub in the same axial plane as said slots and at right angles to said slots for connecting the inner and outer conductors of said first line; means for short circuiting said first line approximately one quarter of a wavelength from said slots; and means for short circuiting said second line approximately one quarter of the air wavelength from said slots.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,519,208 Wheeler Aug. 15, 1950 2,580,678 Hansen et al Jan. 1, 1952 OTHER REFERENCES Abstract Ser. No. 605,417, published November 7, 1950, 640 O. G. 343. 

